1. Field of the Invention
The present invention relates to an abrasive for polishing glass. More particularly, it relates to an abrasive for polishing glass, which is suitable for finish polishing various types of glass materials and which comprises, as the main component, a rare earth oxide containing cerium oxide and is particularly excellent in the durability of a high polishing rate, and a method for evaluating the quality of abrasive grains.
2. Discussion of Background
In recent years, glass materials are used in various applications. Among them, particularly in e.g. a glass substrate for an optical disk or magnetic disk, a glass substrate for a display such as an active matrix type LCD, a color filter for liquid crystal TV, a watch, a desk-top calculator, LCD for camera or a solar cell, a glass substrate for a LSI photomask, or an optical lens or a glass substrate for an optical lens, it is required to polish the surface highly precisely.
Heretofore, as an abrasive to be used for surface polishing of such glass substrates, an abrasive comprising a rare earth oxide, particularly cerium oxide, as the main component, has been used, because cerium oxide as abrasive grains for polishing has a merit in that its efficiency for polishing glass is superior by a few times as compared with zirconium oxide or silicon dioxide. Further, in recent years, a higher quality is required also in the polishing ability of the abrasive. For example, along with an increase in the amount of image information or moving image information, a higher recording density (from about a few tens giga to over 100 giga) is required for built-in HDD (hard disk drive/magnetic disk) in e.g. a personal computer, a DVD recorder or a car navigation, and it is required to polish the substrate surface more precisely and flatly. In HDD, the space between the magnetic surface and the magnetic head flying by so-called CSS (contact start stop) is now at most 0.1 μm.
Further, the abrasive comprising cerium oxide as the main component for polishing glass usually contains a fluorine content for the purpose of improving the polishing performance.
Heretofore, various methods are known as methods for producing abrasives containing a fluorine content.
For example, in a method wherein a bastnaesite concentrate (rare earth fluoride carbonate) is, for example, selectively used as a material initially containing a fluorine content, fluorine is already contained in the concentrate, whereby it is possible to obtain a final product abrasive which also contains fluorine.
Further, a method for adding and incorporating cerium fluoride as a fluorine content to an abrasive, is also proposed (JP-A-6-330025).
On the other hand, a method is also known wherein a rare earth carbonate is used as a starting material and is partially fluorinated by hydrofluoric acid, followed by drying and calcination, to obtain an abrasive which comprises cerium oxide as the main component and which contains fluorine, or as a method wherein no partial fluorination by hydrofluoric acid is carried out, a method is proposed wherein alkali metals, alkaline earth metals and radio active substances are chemically separated to obtain a light rare earth starting material having their content reduced and comprising cerium as the main component, and a rare earth fluoride is added to such a starting material, followed by firing to make cerium oxide as the main component (JP-A-9-183966).
Further, a method which is understood to be an example belonging to the technical scope of the method as disclosed in the above-mentioned JP-A-9-183966, is also proposed. Namely, a method is disclosed in which a rare earth carbonate is used as a starting material, and it is calcined for thermal decomposition into a rare earth oxide, whereupon a rare earth fluoride is added thereto, followed by operation of pulverization, firing, crushing and classifying (JP-A-2002-224949).
Further, it is regarded as preferred that the content of the fluorine content in the abrasive is, for example, within a range of from about 3 to 9 mass %. If the fluorine content is too small, it is not possible to sufficiently change lanthanum oxide to lanthanum oxyfluoride (LaOF) for fixing, whereby the polishing rate tends to be low. The presence of strongly basic lanthanum oxide is likely to cause clogging of a polishing pad during polishing, which adversely affects refreshing by circulation of the abrasive slurry to the polishing surface. On the other hand, if the fluorine content is too much, the excess rare earth fluoride is likely to undergo sintering during firing, such being undesirable (JP-A-9-183966).
Further, with respect to an abrasive comprising cerium oxide as the main component, it has been proposed that an abrasive whereby in the characteristics of the powder X-ray diffraction analysis, the ratio of the peak intensity of rare earth oxyfluoride, etc. to the maximum peak (rare earth oxide) intensity in the X-ray diffraction shows a specific range, is excellent in the polishing characteristics (JP-A-2002-97457 and JP-A-2002-224949).
Further, with respect to the grain diameter distribution of an adhesive, it is generally accepted that when one having a d90/d10 ratio of the average grain diameters in the cumulative grain diameter distribution within a specific range, is used, it is possible to increase the polishing rate and to minimize formation of scratches or the like on the polished surface (JP-A-2002-194334).
However, even if the fluorine content, the powder X-ray diffraction pattern and the particle size distribution, etc. are adjusted to be in the preferred ranges as proposed above, the above-mentioned conventional adhesive comprising cerium oxide as the main component is still not free from the following undesirable phenomenon, and it is not necessarily satisfactory from the viewpoint of the durability of the polishing performance to continuously maintain a high polishing rate while maintaining the quality of the polished surface of the glass.
Here, the basic mechanism in glass polishing and elution of the substrate components will be described as follows.
Namely, as an abrasive for polishing glass, usually, abrasive grains of e.g. cerium oxide type are dispersed in a liquid such as water and used in the form of an abrasive slurry. Further, in the case of continuous and large amount of surface polishing of a glass plate, it is common to use such an abrasive slurry by recycling and at the same time, the amount corresponding to a loss of the abrasive slurry taken out of the system as deposited on the polished glass plate, is supplemented as a fresh slurry from outside the system.
Polishing of glass by a cerium oxide type abrasive is generally regarded as a combination of mechanical polishing and chemical polishing (e.g. “Optical Glass” Tetsuro Izumitani and published by Kyouritsu Shuppan, 1984, p. 114–131, and Lee M. Cook, Chemical Processes In Glass Polishing, Journal of Non-Crystalline Solids, North-Holland 120(1990), p. 152–171). In these literatures, both the mechanical polishing mechanism and the chemical polishing mechanism in glass polishing are described in general.
Firstly, in the aspect of mechanical polishing of glass, the following formula (Preston formula) is introduced with respect to the polishing rate.
Preston FormulaΔH/Δt=Kp*(L/A)*(Δs/Δt)where ΔH/Δt: the height ΔH of the substrate to be polished, which changes in time Δt.
L: Total load
A: Area of the substrate to be polished
Δs: The relative moving distance of the polishing tool on the glass surface, which moves on the surface to be polished.
Kp: Preston constant
Another Expression of the Preston FormulaΔH/Δt=(2E)−1*P*(Δs/Δt)where ΔH/Δt: the height ΔH of the substrate to be polished, which changes in time Δt.
E: Young's modulus of the glass substrate
P: Pressure per area to which the load is exerted
Δs: The relative moving distance of the polishing tool on the glass surface, which moves on the surface to be polished.1/(2E): Preston constant=Kp
As in the above formulae, the polishing rate (ΔH/Δt) based on the mechanical mechanism is expressed by the total load (or pressure), the relative moving speed of the polishing tool and the Young's modulus of the glass substrate.
On the other hand, with respect to the chemical polishing of glass, the following sequential mechanism model has been proposed.
Namely, {circle around (1)} firstly, a soft hydration phase is formed on the glass surface to which the load of abrasive particles is exerted, {circle around (2)} this hydration phase will be weakly bonded to active points on the surface of the abrasive grains and will be abraded off from the glass surface under the load by the abrasive grains, {circle around (3)} thus, the glass hydration phase adsorbed on the surface of the abrasive grains will be hydrolyzed and detached from the abrasive grains, and the glass component (the main component is silica) is dissolved in water as the solvent, and {circle around (4)} a process of adsorption of the dissolved silica on the abrasive grains or reprecipitation of silica will take place.
Here, the phenomenon of the reprecipitation of the silica dissolved in water is disclosed in detail also in the famous Iler's book relating to silica chemistry (Ralph K. Iler, The Chemistry of Silica, John Wiley & Sons, 1979 (reprint edition)). Namely, the polymerization speed of the dissolved silica is high when the dissolved silica concentration is high, and the pH dependency of the liquid is substantial, and particularly in a region where the pH is close to neutral, the polymerization-gelation speed of the dissolved silica is regarded to be high. The silica thus polymerized and gelled, will be reprecipitated in water as the solvent containing abrasive grains, but this hydrogel-state silica (the hydrogel-state silica will hereinafter be referred to as gelled silica) is fine and adhesive and acts as a binder for the abrasive grains. Accordingly, as the polymerized and gelled silica will be accumulated in the liquid along with the progress of polishing, it tends to partially cover the glass surface to be polished in such a form as if it binds the abrasive grains, whereby the abrasive grains will be solidified in such a form to cover the polishing pad, whereby the pad tends to be clogged, thus causing formation of scratches on the surface of the object to be polished. Consequently, it causes an undesirable phenomenon such that the quality of the polished surface or the polishing rate will be deteriorated.
As a method for solving such a problem, it has been proposed to add calcium secondary phosphate or the like to the abrasive grains, so that the abrasive grains will be readily redispersed (JP-A-50-13405). The addition of such component is effective to improve the dispersibility of the abrasive grains, but with respect to the quality of the polished surface, latent scratches are likely to be formed substantially, and such is not fully satisfactory especially for a glass substrate where high precision is required.
Further, as still another method, in an abrasive comprising, as the main component, a rare earth oxide containing cerium oxide, a slurry-state abrasive for polishing glass has been proposed in which cerium fluoride and a small amount of a calcium compound are incorporated (JP-A-6-330025, Japanese Patent No. 2832270).
In this method, the calcium compound dissolved in the abrasive slurry is supposed to provide effects in the state of calcium ions.
Each of the above-mentioned methods is effective to some extent for overcoming the problems, but it is susceptible to an influence of the physical properties of the formed grains delicately different depending upon the preparation method or conditions or selection of the starting material for the production of the abrasive comprising cerium oxide as the main component, whereby from the viewpoint of general applicability, it is difficult to provide the effects constantly, thus providing no essential solution.